Split Reference Model into levels of description

[chiarcos]

The OLiA Reference Model is a monolithic knowledge graph for annotation terminology. However, this makes it relatively large and hard to use, and for a future OLiA releases, we suggest to break into different levels of description (syntax, parts of speech, [morphosyntactic] features, morphology, semantics, discourse) with corresponding sub-URIs, e.g., http://purl.org/olia/pos#Noun, http://purl.org/olia/feats#Singular, http://purl.org/olia/syntax#Subject, http://purl.org/olia/morph#Diminuitive, etc.). Then, the corresponding parts of the current olia-top.owl should be distributed across these, as well.

This breaks downward compatibility, hence milestone 1.0.